Thursday 23 February 2012
The gastrointestinal tract is derived largely from the endodermal germ layer.
During the 3rd week of embryonic development, cephalocaudal and lateral folds of the trilaminar germ disk develop and progressively incorporate parts of the endoderm-lined yolk sac into the body cavity to form a tube-like gut.
By the end of week 3 of gestation, an open connection between the anterior portion of this tube, the foregut, and the amniotic cavity is established at the site of the future mouth.
During early embryonic life, the vitelline or omphalomesen-teric duct provides an open connection between the midgut and the yolk sac.
This connection becomes progressively longer and narrower as gestation proceeds and eventually forms part of the umbilical cord.
By week 10, the communication between the lumen of the midgut and the umbilicus becomes obliterated and soon disappears (e328).The laryngotracheal diverticulum develops from the ven-tral foregut during week 4 of gestation.
Gradual formation of an esophagotracheal septum along the length of the laryngotracheal diverticulum separates the ventral respi-ratory and the dorsal digestive tubes.
During the 2nd month of embryonic life, rapid cellular proliferation within the digestive tube causes a transient par-tial obliteration of the duodenal lumen, the so-called solid stage of development.
Recanalization occurs by week 8 of gestation.
Rapid midgut growth within the relatively small body cavity results in a temporary herniation of the length-ening midgut into the umbilical stalk during weeks 6 to 11.
During this physiologic herniation, the intes-tinal loops rotate counterclockwise, a process that continues as the intestinal loops return to the abdominal cavity during weeks 10 and 11, so that the cecum comes to lie in the right side of the abdomen.
If this orderly process fails to occur or is anomalous, the locations of the small and large intestine, mesentery, and ﬁ xation points of the intestine to the body wall will be abnormal.
The hindgut, or posterior portion of the primitive digestive tube, initially ends posteriorly in thecloaca, separated from superﬁ cial ectoderm by the cloacal membrane.
A transverse ridge, the urorectal septum, grows posteriorly from the umbilical stalk and grad-ually divides the cloaca into a ventral portion, the urogenital sinus, and a dorsal portion, the future rectum, and anus.
This division is normally complete at the end of week 6 of gestation.
The membrane covering the anal canal disappears by week 9, so that communication between the digestive tract and the amniotic cavity is established caudally.